Atrial fibrillation is a common sustained cardiac arrhythmia and a major cause of stroke. This condition is perpetuated by reentrant wavelets propagating in an abnormal atrial-tissue substrate. Various approaches have been developed to interrupt wavelets, including surgical or catheter-mediated atriotomy. It is believed that to treat atrial fibrillation by radio-frequency ablation using a catheter, continuous linear lesions must be formed to segment the heart tissue. By segmenting the heart tissue, no electrical activity can be transmitted from one segment to another. Preferably, the segments are made too small to be able to sustain the fibrillatory process. A preferred technique for treating atrial fibrillation by radio-frequency ablation would be a “branding iron” approach, where a relatively long electrode can be held stationary in good contact with the heart wall while ablation is completed. In this way, a continuous transmural burn may be effected.
U.S. Pat. No. 5,800,428 to Nelson et al. discloses a radio frequency ablation catheter system having a flexible, tubular electrode for creating a continuous linear lesion. The tubular electrode is selectively extendable from the distal end of the catheter. The catheter further comprises mechanisms for remotely manipulating and extending the electrode. However, having an extendable electrode house in the catheter provides less degrees of freedom with respect to the shape, size and length of the tubular electrode. Moreover, the physician has to deal with additional moving and manipulatable parts, adding complexity to the procedure. Further, a retractable electrode can cause contamination because blood or coagulate on the electrode can be pulled into and entrapped inside the catheter. The entrapped coagulate can also affect the ability of the electrode to be further extended and retracted. Accordingly, it would be desirable to provide a catheter design having an electrode for creating linear lesions that overcomes these drawbacks.
U.S. patent application Ser. No. 10/199,525, entitled “Atrial Ablation Catheter and Method for Treating Atrial Fibrillation” discloses a catheter having an ablation assembly bent relative to the catheter body and comprising a generally-straight non-retractable tubular electrode formed of a material having shape-memory having at least one irrigation port through which fluid can pass from the inside to the outside of the electrode. However, in order to create lesions that are sufficiently deep to treat arrhythmias, a relatively high level of radio frequency power is often needed, which can result in coagulum formation and tissue charring. The irrigation ports in the tubular electrode described in U.S. patent application Ser. No. 10/199,525 permit the use of cooling fluid to reduce coagulum formation and tissue charring. However, if the irrigation is not efficiently delivered to the tissue being treated, local hot spots can develop at the tissue/electrode interface. Accordingly, a need exists for an improved catheter containing an elongated electrode that can provide more efficient cooling during ablation.